There is a reason. the VIII driveshaft is VERY long, and a standard aluminum shaft is too flimsy to allow high RPMs inherent with a gear'd VIII. when you get a stock shaft up to speed it will actually start spinning offof it's centerline causing one hell of a vibration. It can be bad enough to break rearends, and I have seen them break tranny extension housings.
the MMx shaft is not only LIGHT, but it's stiff enough to NOT go nuts and vibrate at high speeds, and rugged enough to transmit the torque needed to drag the whale off the line and not break.
worth every penny.
Mike